The invention relates to a fuel supply system for an internal combustion engine with direct fuel injection including a fuel supply pump and downstream thereof a high pressure pump for supplying the fuel to a plurality of injectors and, parallel to the high pressure pump, a hydraulic transmission for providing in the start up phase an increased fuel pressure.
Internal combustion engine with direct fuel injection require already during startup high fuel injection pressures in order to achieve a mixture of the fuel with the combustion air sufficient for ignition of the mixture. The minimum pressure of the fuel for gasoline engines with direct fuel injection is about 50 bar and for diesel engines with direct fuel injection it is about 120 bar.
The fuel supply pumps known from the state of the art cannot provide the necessary fuel injection pressure. The necessary pressure is generally provided by a high pressure pump mounted to the camshaft and is available after about one turn of the camshaft that is two turns of the crankshaft.
Before reaching the above minimum fuel pressure, engines with direct fuel injection cannot start properly. This results in relatively long start up times and detrimentally affects particularly start/stop operation of the engine. In order to provide the minimum fuel pressure early on, the use of pressurized fuel storage arrangements or electrically operated high pressure pumps is known in the art.
DE 38 33 430 A1, for example, discloses a fuel supply system for internal combustion engines with a fuel pump driven by the internal combustion engine and a fuel supply pump in the form of a hydraulic pump which is operated by fuel returned from the engine-driven fuel pump. In the lines interconnecting the fuel pump and the hydraulic pump a pressure storage device with a compensation chamber is arranged. The fuel supply from the hydraulic pump to the pressure storage device is controlled by a valve which is opened when the compensation chamber of the pressure storage device is empty and remains open as long as fuel can flow into the pressure storage device. The fuel returned by way of the pressure line is directed into a chamber of the pressure storage device disposed between the valve and the hydraulic pump or, respectively, the pressure line.
However, with this arrangement, the formation of gas bubbles in the suction line of the hydraulic fuel pump (HDP) cannot safely be avoided. Also, the use of an electric fuel supply or high pressure pump has the disadvantage that these pumps are very expensive and subject to failure, that, in order to provide the necessary fuel pressure, they must be relatively large and that such pumps generate certain noises.
DE 199 52 782 A1 discloses a system for reducing the aerosol contained in the liquid fuel in a fuel storage device. An internal combustion engine operating with the common rail fuel injection principle includes—besides the rail and a plurality of injectors—a fuel storage unit which is connected, via a communication line, to a fuel filter, a fuel supply pump and a high pressure pump.
WO 99/28620 discloses a fuel supply system for internal combustion engines with direct fuel injection including a fuel supply pump and a downstream high pressure pump for supplying fuel via a pipe system from a fuel tank to a plurality of injectors or injection nozzles disposed downstream of the high pressure pump wherein between the fuel supply pump and the plurality of injectors or injection nozzles a hydraulic transmission is provided in parallel with the high pressure pump. The hydraulic transmission is designed for the generation and maintenance of a high fuel pressure in the startup and shut down phases of the internal combustion engine. The fuel supply pump is connected at its inlet side to the fuel tank and with its outlet to the hydraulic transmission and the supply pump is connected by way of a branch line to the high pressure pump.
It is the object of the present invention to provide a fuel supply system for internal combustion engines with direct fuel injection with which a fuel pressure as required for fuel injection can be maintained in the startup and shutdown phases of the internal combustion engine in a particularly simple manner.